Part one of my question is,are there diseases or biological means that can alter an organism's DNA? I know sources such as radiation can mutate DNA, I'm wondering if any disease/virus does so as well.

Second part is, if a host's DNA is mutated, does it affect the host's condition, or only its offspring's? As far as I know, DNA has no impact on a organism's development past the embryonic stage? Does the host still utilize/translate DNA information during its lifetime or does the host only pass it down?

Second and mainly, organisms express genes whole lifetime. Contrary to what you say, it will probably affect the host (probably because it doesn't have to), but probably will not affect the offspring. To affect offspring it would have to alter DNA in gametes (sperm, ovaries).

Viruses insert proviruses into cells. The provirus is the genetic code for that virus. This is how a virus can turn a cell into a virus factory.However, some proviruses can remain latent, so an infected cell will pass on that provirus with the rest of its DNA when it divides. But even latent proviruses can harm cells by messing with the DNA. This DNA tampering can even contribute to cancer. This is why gene therapy, which uses genetically engineered viruses to insert the missing gene, is controversial.